In geological exploration it is frequently desired to perform certain observations or carry out operations over limited segments of a bore hole located very substantial distances beneath the surface. In such cases the relative distances between locations where observations are made are of much greater significance than the actual distances from the surface. By way of example, knowing the actual thickness of an oil containing stratum is of great significance compared with knowing its depth below the surface, when seeking to estimate the potential production which might be available from a field. Sondes or logging tools are lowered into bore holes by cables and measurement of the amount of cable payed out indicates the elevation of the tool within the bore hole. In the case of deep holes, however, vertical oscillation of the tools occurs because of the elasticity of the suspending cable even though damped by bore hole fluids. Moreover, precise measurements of cable lengths when a mile or more of cable has been payed out are virtually impossible. Expansion and contraction of long cables due to temperature also leads to erroneous readings.
Accordingly, it is an object of this invention to provide an apparatus which will produce precise measurements of points at which readings or observations are made relative to each other within selected segments of a bore hole.
A second object is to provide a simple relatively rugged apparatus which is capable of sending electrical signals to the surface for indicating the relative distance between read points within a bore hole segment and which employs a minimum number of conductors in the wireline for operation and signal transmission.
Other objects and advantages of the apparatus of this invention will be apparent to those skilled in the art from the following description.